1. Field of the Invention
This invention relates to the field of monitoring water quality and, more particularly, to monitoring water quality using bivalve mollusks to detect toxicants.
2. Discussion of the Prior Art
Various organisms have been used to aid in the detection of low-concentration waterborne toxins or toxicants (hereinafter both being referred to as “toxicants” in the broadest sense of the word). Systems for testing water quality using fish, for instance, have been known for some time. For example, in U.S. Pat. No. 4,626,992 to Greaves et al, a video camera monitors the swimming of the fish, and toxicity in the water is indicated by significant changes in swimming behavior. Another system of this type is described in U.S. Pat. No. 4,723,511 to Solman et al., wherein electrodes monitor electric currents generated by fish in a tank of water being tested.
Systems which use fish, however, have a number of disadvantages. Usually, the fish are free to move in the system, which introduces ambiguous behavioral variables. The fish also need to be fed which is periodic and unpredictable. Also the fish must be otherwise cared for, resulting in a high maintenance system which is unsuitable for long-term unattended operation.
Systems for monitoring water quality using bivalve mollusks have also been suggested in the prior art. Mollusks are particularly sensitive to some pollutants, such as chlorine or sulphuric acid, to metals like copper or cadmium, and to a wide range of organic compounds such as pesticides and hydrocarbons. These systems usually rely on the simple fact that a bivalve keeps its shells open under normal conditions, but closes its shells when a toxicant is detected.
Depending on the species, bivalve mollusks can be long lived with life expectancies of 60 to 80 years. Because the animals are filter feeders, and therefore self feeding, deriving all their nutrition and oxygen from the ambient water, a single cohort could theoretically be used for many years in an in situ monitoring system. To ensure peak physical condition and maximum sensitivity in detecting toxicants, however, bivalves in the monitoring system should be routinely changed at four- to six-month intervals depending on conditions. Also, if exposed to a toxicant, the bivalves should immediately be replaced with naive animals, i.e. mollusks that have not previously been exposed to toxicants.
Mollusk-based detection systems of the prior art, however, are generally not configured for the ready installation or removal of mollusks, which can vary considerably in size, even within a species. There is consequently substantial work required to install and calibrate the sensor system, which in the prior art is effectively built around the individual mollusk.
In addition, systems that detect the presence of toxicants in response to closure of the shell of the mollusk neglect earlier indications of toxicant presence that precede full closure of the shell. This delays the detection of toxicants, especially in lower concentrations. There is also a potential for false negatives or false positives in prior art systems due to a slow or only partial closure of the shell in the presence of a toxicant, due to diurnal behaviors of the mollusk.